Smart Rocks Deployed!

Posted on: October 13th, 2017 by Leslie_Zucker

The Ashokan Water­shed Stream Man­age­ment Pro­gram and the NYC Depart­ment of Envi­ron­men­tal Pro­tec­tion are work­ing with the U.S. Geo­log­i­cal Sur­vey to study the rate at which larger rocks and cob­ble move down stream chan­nels. Fine sus­pended sed­i­ments travel with stream flows and pose the great­est threat to water qual­ity in the upper Eso­pus Creek. But larger mate­r­ial rest­ing on the chan­nel bed is also sus­pended dur­ing stream flows and then rede­posited down­stream. Streams in equi­lib­rium with their sur­round­ing land­scape accom­plish sed­i­ment trans­port in an orderly fash­ion. Rocks and cob­ble on the stream bed are lifted from their loca­tions in rif­fles and rede­posited down­stream in another shal­low area where rif­fles form. It’s when the chan­nel is dis­turbed or thrown out of bal­ance that the process can go awry. Large sed­i­ment accu­mu­la­tions in the cen­ter of the chan­nel, like those deposited dur­ing Trop­i­cal Storm Irene for exam­ple, can split stream flows and push water against the banks. The banks may con­tain fine sed­i­ments that cloud drink­ing water or gen­er­ate more coarse mate­r­ial that blocks bridges down­stream. For this rea­son stream man­agers would like to know more about the coarse sed­i­ment — when and where it moves, to bet­ter main­tain stream chan­nel stability.

The large sed­i­ment, rocks and cob­ble, trav­el­ing down the chan­nel bed is called “bed load.“
The AWSMP is part­ner­ing with the USGS to test dif­fer­ent tech­niques for mon­i­tor­ing the amount and rate of bed­load move­ment. Bed load mon­i­tor­ing is dif­fi­cult, time inten­sive and expense to pull off. Sev­eral tech­niques being piloted are meant to save time and make the effort more manageable.

The USGS has installed hydrophones at two bridges in the water­shed. The hydrophones are trig­gered when flows rise and record the sounds rocks make hit­ting against each other as tur­bu­lent water car­ries them down­stream. USGS sci­en­tists will parse the data and check it against phys­i­cal sam­ples taken at the bridges at the same time to see if a sound sig­na­ture can be used to quan­tify the sed­i­ment being transported.

RFID Tag on Smart Rock

An RFID-tag was drilled and glued into this rock pulled from the stream chan­nel to allow for radio-tracking later.

A sec­ond approach is to embed RFID (radio-frequency iden­ti­fi­ca­tion device) tags into native rock mate­r­ial and then find these rocks later using a hand-held antenna dur­ing sweeps of the chan­nel after bed-load mov­ing storm events. Another “smart” way to track rocks is to put an accelerom­e­ter into pre-manufactured rocks along with the RFID tags to gather the rate at which rocks move.

Finally, USGS staff will deploy staff to cap­ture sed­i­ment as it flows under the bridges. The bed­load sam­plers are low­ered from the bridge to the chan­nel bed and fill with sed­i­ment that is dumped into buck­ets and later sorted and mea­sured. Sam­pling con­tin­ues as long as prac­ti­cal until stream flows recede!

Bed Load Sampler

USGS sci­en­tist Jason Siemion low­ers the bed load sam­pler from a bridge.

The entire effort depends on hav­ing stream flows strong enough to move most of the sed­i­ment lying on the chan­nel bed. Sur­pris­ingly, almost the entire stream bed moves dur­ing flow events that occur as fre­quently as every sev­eral years on aver­age. And at least some of the bed load moves dur­ing smaller events. The project team is hop­ing for just enough rain to cause bed-load move­ment but not enough to cause any damage!

The study will run through 2019 with results to follow.